1. Field of the Invention
The present invention relates to a mobile communication system such as a second-generation cordless telephone system and, more particularly, to control the transmission power on the mobile station side.
2. Description of the Prior Art
Second-generation cordless telephones (PHS) have been widely used as mobile telephones. In a system using such telephones, one service area is divided into a plurality of zones (see FIG. 2A), and a base station is installed in each zone. A base station and a mobile telephone (to be referred to as a mobile station hereinafter) are connected to each other by radio. When a conventional mobile station is near a base station, speech communication can be performed with a small transmission power. When, however, the mobile station is far from the base station, since speech communication cannot be performed with a small transmission power, the mobile station always transmits signals with a high power. Since the mobile station is driven by a battery, if transmission is always performed with a high power, the battery power is greatly consumed, and a long speech communication enable time cannot be obtained.
Under the circumstances, in the technique disclosed in Japanese Unexamined Patent Publication No. 7-336291, the reception electric field strength of a signal from a base station is measured on the mobile station side, and the transmission power is increased if the reception electric field strength decreases. With this technique, of battery power consumption is suppressed to prolong the service life of the battery.
In the invention disclosed in Japanese Unexamined Patent Publication No. 7-336291, when either the reception electric field strength or the bit error rate decreases, the transmission power is increased. An arrangement designed to control transmission power in accordance with a reception electric field strength will be described with reference to FIGS. 1A and 1B. FIG. 1A is a block diagram plainly showing the basic arrangement of a mobile station. FIG. 1B is a flow chart for explaining transmission power control.
This mobile station transmits/receives signals to/from a base station through an antenna 1. A received signal is sent to a control section 4 through a receiving section 3 to be subjected to communication control in a central processing unit (CPU) 4a, a read-only memory (ROM), and a random access memory (RAM) 4c. This signal is also sent to an electric field strength measuring section 7, in which the reception electric field strength of the signal is measured. In accordance with the control result obtained by the control section 4, a reception speech signal is sent to a receiver section 6, and a transmission speech signal is received through a transmitter section 5. The reception electric field strength measured by the electric field strength measuring section 7 is sent to the power control section 4d in the control section 4 to control the transmission power in a transmitting section 2.
Referring to the flow chart of FIG. 1B, the mobile station measures a reception electric field strength at predetermined time intervals (step 5-1). It is checked on the basis of the measurement result whether it is necessary to change the transmission power (step 5-2).
If it is determined that it is necessary to change the transmission power, the power control section 4d controls the transmission power to decrease the transmission power when the reception electric field strength is high, and to increase the transmission power when the reception electric field strength is low (step 5-3).
If it is determined in step 5-2 that the transmission power need not be changed, or the transmission power is controlled in step 5-3, the flow returns to step 5-1 to measure a reception electric field strength. Control on the transmission power is repeated in this manner.
That is, in the conventional transmission power control technique, the transmission power is controlled by determining the relative distance between a base station and a mobile station on the basis of the reception electric field strength. According to this control technique, as the relative distance between the base station and the mobile station decreases, the transmission power of the mobile station is decreased to prevent wasteful consumption of battery power as compared with a mobile station in which the transmission power is fixed. As a result, the operation time of the battery-driven mobile station can be prolonged.
Such a technique of reducing battery power consumption is effective when base stations (6-1) in zones (6-2) in a service area (6-3) are identical to each other, and the respective base stations (6-1) have the same transmission power, as shown in FIG. 2A. With the recent rapid increase in the number of mobile stations, base stations are in the urgent need of upgrading. The use of large-output base stations, which allow services with a small number of base stations, has been started. The following are the principal objects of the construction of a service area (6-10) using a plurality of base stations having different transmission powers, as shown in FIG. 2B.
The first object is to ensure a traffic in an area (6-4) which requires a high traffic. In practice, a large number of small-diameter zones in which base stations having a relatively small transmission power (to be referred to as low-transmission-power base stations (6-5)) are arranged in the area (6-4) to increase the density of base stations, thereby ensuring the traffic.
The second object is to upgrade base stations and broaden the overall service area by using a relatively small number of base stations to form the service area of an area (6-8) in which the density of mobile stations is low so the priority tends to be low in terms of upgrading of base stations, and a much high traffic is not required. In practice, a base station having a relatively high transmission power (to be referred to as a high-transmission-power base station (6-9) hereinafter) and forming a zone having a large radius is installed to decrease the number of base stations required to construct the service area, thereby upgrading base stations.
In an area (6-6) which requires an intermediate traffic, base stations having an intermediate transmission power (to be referred to as intermediate-transmission-power base stations (6-7) hereinafter) and forming a zone having a radius corresponding to the traffic are installed. In this manner, a plurality of types of base stations having different transmission powers are prepared to form the service area in consideration of the necessary traffics and the cost in base station upgrading. Current second-generation cordless telephone systems and the like form service areas by using a plurality of types of base stations having different transmission powers.
In such a state in which different transmission powers are set, the distance between a base station and a mobile station cannot be determined from a reception electric field strength, and hence the communication quality cannot be maintained by the conventional transmission power control technique.
As described above, the mobile station transmission power control method used in the conventional mobile communication system is based on the assumption that a uniform transmission power is set in the respective base stations. The relative distance between a mobile station and a base station is therefore determined on the basis of only a reception electric field strength in the mobile station. In a current mobile communication system, however, the service area is constituted by a plurality of types of base stations having different transmission powers. For this reason, the relative distance between a mobile station and a base station cannot be accurately calculated by the conventional mobile station transmission power control method. Speech communication is therefore disabled in some places.
Assume that a base station having a high transmission power is additionally installed in the conventional service area constituted by base stations having the same transmission power (to be referred to as standard base stations hereinafter) to broaden the service area. In this case, although the transmission power of a mobile station is properly controlled in the zone of a standard base station, the following problems are posed when the mobile station is located in the zone of the high-transmission-power base station.
Assume that a mobile station is at the same distance from the high-power-transmission base station as the distance at which the mobile station is far from a standard base station and an electromagnetic wave transmitted therefrom is weak so that the mobile station must perform transmission with the maximum transmission power. In such a place, the strength of the electric field received by the mobile station is larger than that received from the standard base station. For this reason, the mobile station determines that the electric field has a sufficiently high strength, and decreases the transmission power. The transmission performance of the mobile station, however, remains unchanged. If, therefore, the transmission power is decreased at the distance at which the standard base station can barely receive a signal transmitted with the maximum transmission power, even the high-transmission-power base station cannot receive the signal.
The above problem is based on the fact that the relative distance between a mobile station and a base station cannot be determined from a reception electric field strength alone. That is, the conventional mobile station transmission power control method is not effective in a service area in which base stations having different transmission powers are installed.
In general, each base station has a code for identifying itself as a base station ID, and transmits it to a mobile station. If, therefore, the base station IDs are identified to determine the transmission powers of the respective base stations on the mobile station side, and control is performed in accordance with the respective transmission powers, the above problem is solved. For this purpose, the mobile station must have information indicating the transmission powers corresponding to the respective base station IDs. This base station information is stored in the memory in the mobile station. A large memory capacity is, however, required to store all base station information in the memory. In addition, since the contents of the memory in the mobile station side cannot be updated, this method cannot cope with the installation of a new base station.